The use of HTc superconductive multifilament strands with AC requires good decoupling of the HTc superconductive filaments making it up in order to limit energy losses due to induced currents.
It is known to make HTc multifilament strands by the "powder in tube" technique. That consists in filling a billet with powder reagents that are suitable, after heat treatment, for transforming into a superconductive material, and in particular into a material of the HTc ceramic type.
The billet is then closed under a vacuum and drawn down, after which it is put into a bundle in a new billet itself in turn closed under a vacuum and then drawn down. The resulting multifilament strand may be subjected to the same steps, and so on until a desired number of filaments per unit area has been obtained.
The strand made in this way is then put into its final form, e.g. by rolling and/or twisting, and is then subjected to heat treatment to transform its powder reagents.
The material constituting the billets must be sufficiently ductile to be capable of withstanding the various drawing-down and rolling stages, and its composition must be inert or at least without consequence for the heat treatment that transforms the powder reagents into a superconductive phase. It is known that silver can be used as the material constituting the billets.
However, silver is a material that is very highly conductive at the operating temperatures of HTc superconductors. As a result there is practically no electrical decoupling between the filaments.
It is known that Ag can be doped with impurities of the Pd or Au type to 1% or 2%. That technique makes it possible to gain two decades in resistivity at 20 K.
That technique can also be used for applications at 77 K. However, the Ag/Pd alloy is expensive which makes it economically inconceivable in mass production applications.
In addition to increasing the resistivity of the silver-based matrix, it is also known to twist the conductor at a very small pitch with a very small filament diameter. However the resulting decoupling is not sufficient in most AC applications.